1. Field of the Invention
This invention relates in general to an impact test apparatus, and specifically to dropped-weight testing assembly for impact testing of composite materials and structures by vertically dropping a weight from a predetermined height.
2. Description of the Related Art
The use of composite materials for structural applications is increasing due to cost and weight advantages provided by composites. For example, composite materials find applications as structural parts of aircrafts. However, test data have shown that the compression strength of certain composite structures can be significantly reduced by low speed impact damage, e.g., tools dropped by maintenance and assembly personnel. Such low speed impact damage is often undetectable by visual inspection. The sensitivity of the compression strength of composite structures to low speed impact damage has led to the development of more damage tolerant materials and to the development of special damage tolerance design criteria for composite structures.
In order to accurately determine the effect of low speed impacts on composite structures and materials, dropped-weight testing apparatus and procedures have been developed. These apparatus and procedures call for the dropping of a weight, of given size and material, onto a specimen from a given height to generate a certain impact response. In general, investigators have used either a free-fall dropped-weight or a pendulum apparatus to impact the specimen. The present invention relates to a vertical free-fall dropped-weight testing apparatus.
Currently, two types of dropped-weight machines are used by researchers for impacting test specimens. One is a sophisticated dropped-weight system for screening materials in a manual or automated manner for damage tolerance characteristics. In such systems, the weight is typically guided by two or four guide bars and hoisted by a motor operated cable system. Thereafter, the weight is released and descends to impact the test specimen. To prevent multiple impacts, the dropped-weight is arrested by means of a hydraulic or pneumatically actuated system after rebounding from the test specimen. Such machines are typically very expensive and can only test small specimens.
The second type of dropped-weight test apparatus is a basic machine where the weight is moved manually up a frame to a predetermined height and released by pulling a locking-pin mechanism. After impact, the weight is manually caught as it rebounds from the test specimen. Not only is such an operating procedure unsafe, but the impact location cannot be accurately controlled, making for wide discrepancies in the accuracy of acquired test data.
U.S. Pat. No. 3,106,834 to Parstorfer discloses a high acceleration impact shock testing apparatus which comprises a tester carriage slidably constrained for movement between parallel guide members. The tester carriage includes a specimen carrier and an impact member. A slidably disposed rebound stop member also carried by the carriage, is laterally deflected upon impact to frictionally engage the stop member with the guide members thereby terminating movement of the carriage at the peak of its rebound from the impact. Shock, strain or impact registering transducers may be carried by the specimen carrier for registering suitable outputs indicating shock, strain, impact, etc. As the specimen travels with the impact member, the size of the specimen is limited.
U.S. Pat. No. 3,103,116 to Kohli discloses a shock testing machine having a brake mechanism for preventing multiple rebounds of a dropped-weight assembly. The brake mechanism comprises a pneumatic brake which is actuated by a control circuit to prevent relative movement between the weight assembly and guide rods.
U.S. Pat. No. 3,209,580 to Colby discloses a device for preventing multiple rebounds of an impacting mass of a free-falling shock testing machine. The device for preventing multiple rebounds of the impacting mass comprises an air cylinder including a reciprocative piston.
U.S. Pat. No. 4,505,362 to Layotte et al. discloses a device for avoiding multiple bounces of a vertically falling mass after a first impact. The device comprises at least one deformable element secured to a guide and a control is adapted to press the deformable element against a lateral wall of the mass after its bounce and to secure the latter in position.
U.S. Pat. No. 2,590,486 to Aubert discloses a scleroscope having a clutching mechanism for stopping a test plunger at a maximum height of rebound. A movable hammer plunger of a magnetic material is slidably positioned in a guide passage. The hammer plunger is dropped from a predetermined height onto a surface to be tested. A magnetic wedge in the frame acts as a clutching or braking mechanism to stop downward movement of the plunger after it has reached its maximum rebound height.
U.S. Pat. No. 2,496,420 to Stern discloses a drop weight test apparatus having two parallel vertical guides, a top plate at the top of the guides, a bottom plate, a weight slidably mounted on the guides, a supporting plate slidably mounted on the guides and bearing a hydraulic velocity reducer. A specimen is mounted for testing beneath the hydraulic velocity reducer by applying a compressive force to it. Between the specimen and the top plate is mounted a piezoelectric strain gage. The weight is raised to a desired height by means of a line. The line is then released so that weight falls, striking the top of the hydraulic velocity reducer. The impact is transmitted through the velocity reducer to the test specimen and the strain gage. The shock or concussion of the impact is absorbed by a resilient layer.
U.S. Pat. No. 2,475,614 to Hoppmann, II, et al. discloses an apparatus for electrically measuring strain applied in testing strength of materials by impact testing. The testing apparatus comprises a tower extending upwardly from an anvil to a predetermined height. A specimen assembly comprises a hammer or tup and a weight attached to respective opposite sides of a specimen of a material being tested, the specimen being enclosed inside a spacer. The specimen is dropped from a predetermined height in the tower and is guided by rails of the tower toward the anvil with the weight in advance and the hammer trailing. The weight is contoured to enter an aperture of the anvil without interference and the hammer is contoured to extend laterally and overhang the sides of the aperture, whereby it strikes the anvil. A weight bar couples the weight to the specimen. A series of strain gages are fastened on the sides of the bar.
U.S. Pat. No. 161,737 to Beardslee discloses a testing apparatus comprising an iron hammer mounted between two vertical rods, an anvil and a hoisting/release assembly. The hammer is raised by the hoisting assembly. A specimen is then placed on the anvil and the hammer is released to strike the specimen.
U.S. Pat. No. 3,998,090 to Wislocki discloses a dropped hammer soil compactor including a limit mechanism used to control the distance of the hammer drop. The limit mechanism includes upper and lower limit valves. The upper limit valve is fixed, but the lower limit valve is vertically adjustable.
U.S. Pat. No. 4,640,120 to Garritano et al. discloses an impact testing apparatus including a weighted dart which is dropped from a prescribed height to penetrate a test specimen. A force transducer is located at the tip of the dart for providing impact force information as the tip penetrates the test specimen.
U.S. Pat. No. 1,901,460 to Lewis discloses testing apparatus adapted to test the strength of the materials by subjecting the material to a number of impacts imposed by a weight dropped from predetermined heights. A mechanical inertial switch on the impacting weight is used to prevent multiple strikes. Upon striking the test specimen, the switch is propelled downward allowing lifting lugs to engage a central guide rod. The lifting rod and latch pins form a ratchet mechanism which allows motion of the weight upward, but restrains any motion downward.